The leukocyte group 5 antigenic system has two co-dominant alleles (ta and 5b) which segregate independently of HLA and are expressed on a variety of cells and tissues including lymphocytes, platelets, spleen, tonsil, kidney and placenta. We made an initial discovery that the 5a allele of this system has a striking association with acute lymphoblastic leukemia (ALL). This is the strongest assoication yet found of an antigen with human leukemia. In subsequent studies, we also have found that the group 5a allele is associated with chronic lymphocytic leukemia and lymphocytic lymphoma. We propose to: 1. extend our investigation on the association of the group 5a antigen with ALL, 2. produce monoclonal antibodies to the group 5 antigens which may be useful for research and screening programs, 3. determine if the group 5a antigen in recently diagnosed patients is prognostic patients is prognostic of response to therapy, 4. investigate the basis of a possible relationship between the group 5a antigen and survival following bone marrow graft rejection, 5. determine if interaction of the group 5 antigens and certain HLA antigens correlate with the development or course of ALL and 6. obtain insight into the development of lymphoid malignancies by assigning the group 5 locus to a particular linkage group or chromosome; a product of a locus linked to the group 5 locus, rather than the 5a antigen itself, actually may be involved in the development of lymphoid malignancy.